1. Field of the Invention
First, the present invention relates to a vibration gas density meter in which anti-vibration properties are improved, no specific anti-vibration measures are necessary, and the intrinsically safe explosion-protected construction is easily adopted.
Second, the present invention relates to a vibration gas density meter in which its temperature characteristics are improved, no specific heat-insulating measures are necessary, and the intrinsically safe explosion-protected construction is easily adopted.
Third, the present invention relates to a vibration gas density meter in which its ambient temperature characteristics are improved.
Fourth, the present invention relates to a vibration gas density meter in which its sensitivity is improved and the pressure loss is less.
2. Description of the Prior Art
FIG. 1 shows a drawing to illustrate the configuration of an embodiment of the prior art generally used to date. This drawing is, for example, shown on page 4 of the catalog titled "DG8 type Gas Density Meter," published on Jul. 15, 1990, by Yokogawa Electric Corporation.
FIG. 2 shows an A--A cross sectional drawing for FIG. 1, and FIGS. 3 and 4 are drawings for explaining operations of the embodiment indicated in FIG. 1.
In FIG. 1 and FIG. 2, numeral 1 shows the center block provided to avoid requiring a large flow of a sample gas.
Numeral 2 shows a resistance thermometer embedded in center block 1.
Numeral 3 shows an inside cylinder incorporating resistance thermometer 2 concentrically.
Numeral 4 shows an outside cylinder incorporating inside cylinder 3 concentrically.
Numeral 5 shows a thin wall cylindrical resonator provided between inside cylinder 3 and outside cylinder 4 concentrically.
Numeral 6 shows vibratory elements to excite cylindrical resonator 5. In this case, piezo-electric elements are used.
Numeral 7 shows a case incorporating outside cylinder 4.
Numeral 8 shows a sleeve to fix one end of outside cylinder 4 to case 7. In this case, rigid tetra-fluoro-ethylene (teflon) is used for sleeve 8.
Numeral 9 shows the O-ring 1 that seals the gap between outside cylinder 4 and sleeve 8.
Numeral 11 shows the O-ring 2 that seals the gap between sleeve 8 and case 7.
Numeral 12 shows a piece of vibration-proof rubber through which the other end of outside cylinder 4 is fixed to case 7.
Numeral 13 shows a ring-shaped inside flow way of the resonator that is provided inside bottom block 14 of outside cylinder 4 and that supplies a sample gas to the inner cylindrical surface of cylindrical resonator 5.
Numeral 15 shows the outside flow ways of the resonator that supply a sample gas to the outer cylindrical surface of cylindrical resonator 5. These outside flow ways of the resonator are provided through bottom block 14 of outside cylinder 4 and are arranged on a circumference at the outside of inside flow way of resonator 13 avoiding vibratory elements 6. In this case, four flow ways are provided.
In the above configuration, as shown in FIGS. 3 and 4, the sample gas enters inside cylinder 3, turns back at the lower end of inside cylinder 3, and enters bottom block 14 of outside cylinder 4. The sample gas is then branched to inside flow way of resonator 13 and the outside flow ways of resonator 15. The sample gas then passes along the inner and outer cylindrical surfaces of cylindrical resonator 5 and flows out of outside cylinder 4.
The density of a sample gas is determined by measuring the resonance frequency of cylindrical resonator 5 by utilizing the fact that the resonance frequency of cylindrical resonator 5 varies with the density of gas around cylindrical resonator 5.
However, in such a device,
(1) Outside cylinder 4 is supported by case 7 through sleeve 8 and vibration-proof rubber 12.
Accordingly, outside cylinder 4 was provided with general anti-vibration measures against case 7. However, since sufficient vibration-resistant construction was not considered, vibration propagated from case 7 or the gas piping for introducing a sample gas. For this reason, if vibration disturbance was strong, additional anti-vibration measures were necessary.
In addition, if vibration of the piping was strong, metal pipes could not be used but pipes made of a material of polyethylene or the like had to be used, and so the gases that could be measured were limited, which was a problem.
Also, since outside cylinder 4 was electrically insulated from case 7, an intrinsically safe explosion-protected construction could be obtained but that construction was complex and the manufacturing cost was high.
(2) Outside cylinder 4 was supported by case 7 through sleeve 8 and vibration-proof rubber 12.
Accordingly, outside cylinder 4 was provided with general heat insulation measures against case 7. However, since the heat-insulated construction sufficient to gain high sensitivity was not considered, heat was conducted from case 7 or the gas piping for introducing a sample gas. For this reason, if ambient temperature changes are violent, specifically if good transient characteristics against sudden temperature changes were to be achieved, additional heat-insulating measures were necessary.
In addition, if ambient temperature changes were violent, heat-insulating measures were necessary, such as housing the meter in a constant temperature oven, placing it in anti-freeze solution, or installing it in an air-conditioned room and others. Such were the problems.
Also, since outside cylinder 4 was electrically insulated from case 7, an intrinsically safe explosion-protected construction could be obtained but that construction was complex and the manufacturing cost was high.
(3) Resistance thermometer 2 for temperature compensation is embedded in center block 1. The sample gas, after passing through the gap between center block 1 and inside cylinder 3, reaches the inner and outer cylindrical surfaces of cylindrical resonator 5.
Since center block 1 consists of metal, its heat capacity is large and so the temperature of center block 1 itself is less affected by transient changes due to changes in external atmospheric temperature and so forth.
While, because the heat capacity of the sample gas is small, heat is exchanged between the sample gas and center block 1 when the sample gas passes through the gap between inside cylinder 3 and center block 1. When the sample gas reaches cylindrical resonator 5, the temperature of the sample gas attains approximately the temperature of center block 1.
Accordingly, resistance thermometer 2 is used to compensate the temperature for the sample gas for the reason that, although resistance thermometer 2 measures the temperature of center block 1, the temperature of the sample gas is also equivalent to that of center block 1.
However, in such a system, if a higher sensitivity-vibration gas density meter is required, its temperature characteristics also need to be more stable and the above-described construction is not appropriate.
(a) That is, due to thermal inflow through outside cylinder 4 from case 7, there occurs a difference between the temperature of cylindrical resonator 5, i.e. the sample gas, and the temperature measured with resistance thermometer 2 in center block 1. PA1 (b) In addition, since resistance thermometer 2 is located in center block 1, it cannot detect the change in the temperature of cylindrical resonator 5 and the sample gas for thermal inflow from outside due to the large heat capacity of center block 1. PA1 (a) Four outside flow ways of resonator 15 are provided on a circumference at the outside of the inside flow way of resonator 13. PA1 (b) Assuming that the gas density meter is directly inserted into the piping for measurement, it has been designed such that the sample gas inlet and outlet are located near to each other. However, this cannot but turn back the stream of sample gas, increasing the pressure loss. PA1 (c) Since center block 1, inside cylinder 3, cylindrical resonator 5 and outside cylinder 4 are assembled concentrically, the construction is complicated and the machining and assembling costs are high. PA1 the first and second elastic elements that are provided between both end faces of the cylinder block and the case respectively, and through which the cylinder block is supported by the case in the direction of cylindrical axis, sealing the gaps between the cylinder block and the case as well, and PA1 the third and fourth elastic elements that are provided between both parts of the outer cylindrical surface on both end face sides of the cylinder block and the case respectively and through which the cylinder block is supported by the case in the radial direction, sealing the gaps between the cylinder block and the case as well. PA1 an introduction pipe, one end of which is connected to the sample gas introducing port side of the cylinder block, the other end of which is connected to the case, and which introduces the sample gas into the cylinder block as well as prevents heat transfer from the case to the cylinder block by having a predetermined length. PA1 first and second concavities provided at both end faces of the case respectively, PA1 first and second elastic elements that are provided between the above first and second concavities and the cylinder block respectively and through which the cylinder block is supported by the case in the direction of cylindrical axis, sealing the gaps between the first and second concavities and the cylinder block as well, and PA1 third and fourth elastic elements that are provided between both parts of the cylindrical surface on both end face sides of the cylinder block and the first and second concavities respectively and through which the cylinder block is supported by the case in the radial direction, sealing the gaps between the cylinder block and the first and second concavities as well. PA1 outside flow ways of the resonator provided through the bottom block of the cylinder block by a predetermined number so that a stagnant part of the sample gas is not generated when the sample gas is supplied to the outer cylindrical surface of the cylindrical resonator, PA1 a sample gas introduction way, one end of which is provided at the case on the bottom block side of the cylinder block and the other end of which is communicated with the predetermined number of outside flow ways of the resonator and the inside flow way of the resonator, and PA1 a sample gas discharge way through which the sample gas is discharged, one end of which is provided at the case on the side opposite to the bottom block of the cylinder block and the other end of which is communicated with a port on the above described side of the cylinder block.
For the above reasons, more accurate temperature compensation cannot be obtained in measuring the density of the sample gas.
(4)
However, if a higher sensitivity-vibration gas density meter is to be developed, it is clear that stagnant parts of the sample gas generated between each two of the four outside flow ways of resonator 15 have an adverse effect on the response characteristics of cylindrical resonator 5.
If the pressure loss increases, the sample gas used for density measurement is vented into the atmosphere or the like as exhaust gas due to the pressure drop. This goes against the recent trend toward protection of the global environment.
In addition, the stream of the sample gas is lengthened corresponding to the length of resistance thermometer 2 to obtain the effective temperature compensation by temperature detection with resistance thermometer 2, but this also causes the pressure loss.